diff --git a/libmysql/libmysql.c b/libmysql/libmysql.c
index febae456fb2..f86435712dc 100644
--- a/libmysql/libmysql.c
+++ b/libmysql/libmysql.c
@@ -696,7 +696,8 @@ mysql_debug(const char *debug __attribute__((unused)))
 #else
     {
       char buff[80];
-      strxnmov(buff,sizeof(buff),"libmysql: ", env);
+      buff[sizeof(buff)-1]= 0;
+      strxnmov(buff,sizeof(buff)-1,"libmysql: ", env, NullS);
       MessageBox((HWND) 0,"Debugging variable MYSQL_DEBUG used",buff,MB_OK);
     }
 #endif